Since the cruise control won't engage until the vehicle speed reaches 25 mph anything under that is ruled out. The faster you go the more resistance in everything, tires, wind, engine friction, transmission friction, etc. The more resistance, the less mpg. So I suggest 25 mph in the lowest numerical gear ratio the engine will smoothly move the car. Lugging along with the engine struggling to keep up a 25 mph speed probably isn't as good as a gear where the engine can move the car steadily and w/out extreme effort.

Just stick me in the gear head, know it all section, my wife does. hwy61